The Early History of Chemistry Atoms, Molecules,...
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Atoms, Molecules, and Ions Chapter 2 The Early History of Chemistry The Early History of Chemistry Before 16th Century – Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold 17th Century – Robert Boyle: First “chemist” to perform quantitative experiments 18th Century – George Stahl: Phlogiston flows out of a burning material. – Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.” Law of Conservation of Mass Law of Conservation of Mass Discovered by Antoine Lavoisier Mass is neither created nor destroyed Combustion involves oxygen, not phlogiston Other Fundamental Chemical Laws Other Fundamental Chemical Laws A given compound always contains exactly the same proportion of elements by mass. Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine. Law of Definite Proportion 35.45 g Cl x 4 = 11.83 g Cl CCl 4 12.01g C 1.000 g C Other Fundamental Chemical Laws Other Fundamental Chemical Laws When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. The ratio of the masses of oxygen in H 2 O and H 2 O 2 will be a small whole number (“2”). Law of Multiple Proportions H 2 O 8.0 g O , H 2 O 2 16.0 g O , so 16.0 g O = 2.0 1.0 g H 1.0 g H 8.0 g O Dalton’s Atomic Theory (1808) Dalton’s Atomic Theory (1808) ° Each element is made up of tiny particles called atoms. ± The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.
Transcript of The Early History of Chemistry Atoms, Molecules,...
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Atoms, Molecules,and Ions
Chapter 2
The Early History of ChemistryThe Early History of Chemistry
Before 16th Century– Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold
17th Century– Robert Boyle: First “chemist” to perform quantitative experiments
18th Century– George Stahl: Phlogiston flows out of a burning material.– Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.”
Law of Conservation of MassLaw of Conservation of Mass
Discovered by Antoine Lavoisier
Mass is neither created nor destroyed
Combustion involves oxygen, not phlogiston
Other Fundamental Chemical LawsOther Fundamental Chemical Laws
A given compound always contains exactly the same proportion of elements by mass.
Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.
Law of Definite Proportion
35.45 g Cl x 4 = 11.83 g Cl CCl412.01g C 1.000 g C
Other Fundamental Chemical LawsOther Fundamental Chemical Laws
When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).
Law of Multiple Proportions
H2O 8.0 g O , H2O2 16.0 g O, so 16.0 g O = 2.0 1.0 g H 1.0 g H 8.0 g O
Dalton’s Atomic Theory (1808)Dalton’s Atomic Theory (1808)
Each element is made up of tiny particles called atoms.
The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.
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Dalton’s Atomic Theory(continued)
Dalton’s Atomic Theory(continued)
Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.
Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.
Avogadro’s Hypothesis (1811)Avogadro’s Hypothesis (1811)
• 5 liters of oxygen• 5 liters of nitrogen
• Same number of particles!
At the same temperature and pressure, equal At the same temperature and pressure, equal volumes of different volumes of different gasesgases contain the same contain the same number of particles.number of particles.
Figure 2.4: A representation of some of Gay-Lussac's experimental results on combining gas volumes.
Figure 2.5: A representation of combining gases at the molecular level.
The spheres represent atoms in the molecules.
2 H2 (g) + O2 (g) º 2 H2O (g)H2 (g) + Cl2 (g) º 2 HCl (g)
Early Experiments to Characterize the AtomEarly Experiments to
Characterize the Atom
J. J. Thomson - postulated the existence of electrons using cathode ray tubes.
Ernest Rutherford - explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.
Figure 2.7: A cathode-ray tube. The fast-moving electrons excite the gas in the tube, causing a glow between the
electrodes.
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Figure 2.8: Deflection of cathode rays by an applied electric field.
Figure 2.5: The cathode-ray beam travels right to left.Photo courtesy of James
Scherer.
Figure 2.5: The beam of negative particles bends downward.Photo courtesy
of James
Scherer.
Figure 2.5: The magnet is turned around and the beam bends in opposite direction.Photo courtesy of
James Scherer.
– In 1909, U.S. physicist, Robert Millikan had obtained the charge on the electron. (see Figure 2.6)
Atomic Theory of Matter• The Structure of the Atom
– These two discoveries combined provided us with the electron’s mass of 9.109 x 10-31 kg, which is more than 1800 times smaller than the mass of the lightest atom (hydrogen).
– These experiments showed that the electron was indeed a subatomic particle.
Figure 2.10: A schematic representation of the apparatus
Millikan used to determine the charge on the electron.
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Figure 2.9: The plum pudding model of the atom.
Figure 2.12: Rutherford's experiment on
α-particle bombardment of metal foil.
Figure 2.13: (a) The expected results of the metal foil experiment if
Thomson's model were correct. (b)Actual results.
The Modern View of Atomic Structure
The Modern View of Atomic Structure
electrons
protons: found in the nucleus, they have a positive charge equal in magnitude to the electron’s negative charge.
neutrons: found in the nucleus, virtually same mass as a proton but no charge.
The The atomatom contains:contains:
Figure 2.14: A nuclear atom viewed in cross section. Note that this drawing is not to scale.
1 atomic mass unit (or Dalton) = 1/12 of a 12C atom or 1.66054 x 10-27 kg. You cannot calculate the absolutemass of a 12C nucleus by adding the masses of 6 protons and 6 neutrons - the mass that you calculate will be greaterthan (12)(1.66054 x 10-27 kg). While mass is conserved in chemical reactions it is not conserved in nuclear reactionsand formation of the carbon nucleus from protons & neutrons results in a small mass loss (see Chapter 18, your text.)
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Figure 2.15: Two isotopes of sodium. Both have eleven protons and eleven electrons,
but they differ in the number of neutrons in their nuclei.
The Chemists’ Shorthand: Atomic Symbols
The Chemists’ Shorthand: Atomic Symbols
K ← Element Symbol39
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Mass number →
Atomic number →
For a neutral K atom
Figure 2.14: A modern form of the periodic table.
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Periodic TablePeriodic TableElements classified by:
properties
atomic number
Groups (vertical)1A = alkali metals2A = alkaline earth metals7A = halogens8A = noble gases
Periods (horizontal)
IonsIons
Cation: A positive ionMg2+, NH4
+
Anion: A negative ionCl−, SO4
2−
Ionic Bonding: Force of attraction between oppositely charged ions.
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Monatomic Ions
Elements which have an excess or deficit of electrons
Examples: Sodium ion has 11 protons and on 10 electrons. The net charge on the atom is 1+ (obtained by adding the charge of the protons and electrons)We would write the symbol for sodium ion orNa+ 23Na+
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What would be the symbol of an element withatomic number 17 and 18 electrons?
35Cl-17
– Most of the main group metals form cations with the charge equal to their group number.
Chemical Substances; Formulas and Names
• Rules for predicting charges on monatomic ions
– The charge on a monatomic anion for a nonmetal equals the group number minus 8.
– Most transition elements form more than one ion, each with a different charge. (see Table 2.5)
The Chemists’ Shorthand:Formulas
The Chemists’ Shorthand:Formulas
• Chemical Formula:• Symbols = types of atoms• Subscripts = relative numbers of each type of
atom CO2
• Structural Formula:• Individual bonds are shown by lines.
O=C=O
– The formula of an ionic compound is written by giving the smallest possible whole-number ratio of different ions in the substance.
Chemical Formulas; Molecular and Ionic Substances
• Ionic substances
– The formula unit of the substance is the group of atoms or ions explicitly symbolized by its formula.
NaCl CaCl2
– When an atom picks up extra electrons, it becomes a negatively charged ion, called an anion.
Chemical Formulas; Molecular and Ionic Substances
• Ionic substances
– An atom that loses electrons becomes a positively charged ion, called a cation.
– An ionic compound is a compound composed of cations and anions held together by ionic bonds..
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Figure 2.19: Sodium metal reacts with chlorine gas to form solid sodium
chloride.
2Na (s) + Cl2 (g) º 2 NaCl (s)
Chemical BondsChemical Bonds
•The forces that hold atoms together in compounds. Covalent bonds result from atoms sharing electrons.
•Molecule: a collection of covalently-bonded atoms.
Figure 2.17: Molecular models of some elementary
substances.
An elemental substance is composed of molecules containingatoms from only one element.
Figure 2.25: Representation of the reaction of methane with
oxygen.
CH4 (g) + 2 O2 (g) º CO2 (g) + 2 H2O (g)
– A molecule is a definite group of atoms that are chemically bonded together – that is, tightly connected by attractive forces.
Chemical Formulas; Molecular and Ionic Substances
• Molecular substances (see Figure 2.16)
– A molecular substance is a substance that is composed of molecules, all of which are alike.
– A molecular formula gives the exact number of atoms of elements in a molecule.
– Structural formulas show how the atoms are bonded to one another in a molecule.
Figure 2.16: Molecular and structural formulas and molecular models.
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Figure 2.16: The structural formula for methane.
Figure 2.18: Ball-and-stick model of methane.
Figure 2.17: Space-filling model of methane. This type of model shows both the relative sizes of the atoms in the molecule and their
spatial relationships.
Chemical Formulas; Molecular and Ionic Substances
• The chemical formula of a substance is a notation using atomic symbols with subscripts to convey the relative proportions of atoms of the different elements in a substance.
– Consider the formula of aluminum oxide, Al2O3. This formula implies that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2:3.
– Most ionic compounds contain metal and nonmetal atoms; for example, NaCl.
Chemical Substances; Formulas and Names
• Ionic compounds (Type I binary compounds
– You name an ionic compound by giving the name of the cation followed by the name of the anion.
– A monatomic ion is an ion formed from a single atom.
Naming CompoundsNaming Compounds
1. Cation first, then anion
2. Monatomic cation = name of the elementCa2+ = calcium ion
3. Monatomic anion = root + -ideCl− = chloride
CaCl2 = calcium chloride
Binary Ionic Compounds:Binary Ionic Compounds:
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Figure 2.22: The common cations and anions
Naming Compounds(continued)
Naming Compounds(continued)
metal forms more than one cation (transition metals)use Roman numeral in name
PbCl2
Pb2+ is the cation
PbCl2 = lead (II) chloride
Binary Ionic Compounds (Type II):Binary Ionic Compounds (Type II):
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Crystals of copper(II) sulfate. Various chromium compounds dissolved in water. From left to right; CrCl2, K2Cr2O7, Cr(NO3)3, CrCl3, K2CrO4.
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– A polyatomic ion is an ion consisting of two or more atoms chemically bonded together and carrying a net electric charge.
– Table 2.6 lists some common polyatomic ions. Here a few examples.
Chemical Substances; Formulas and Names
• Polyatomic ions
nitrite NO
nitrate NO
2
3−
−
sulfite SO
sulfate SO2
3
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−
−
Figure 2.20: Ball-and-stick models of the ammonium ion and the
nitrate ion.
Polyatomic Ions You Should Know
• NH4+ - Ammonium
• OH- - Hydroxide• CN- - Cyanide• SO4
2- - Sulfate• HSO4
- - bisulfate• ClO4
- - Perchlorate• NO3
- - Nitrate• NO2
- - Nitrite
• O22- - Peroxide
• PO43- - Phosphate
• HPO42- - monohydro-
gen phosphate• H2PO4
- - dihydrogen phosphate
• CO32- - Carbonate
• HCO3- - Bicarbonate
More Practice
• Na2SO4 Na2SO3
Sodium Sulfate Sodium Sulfite• AgCN Cd(OH)2
Silver Cyanide Cadmium Hydroxide• Ca(OCl)2 KClO4
Calcium Hypochlorite Potassium Perchlorate
Naming Compounds(continued)
Naming Compounds(continued)
Compounds between two nonmetalsFirst element in the formula is named first.Second element is named as if it were an anion.Use prefixesNever use mono- for the first element in
formula
P2O5 = diphosphorus pentoxide
Binary molecular compounds (Type III):Binary molecular compounds (Type III):
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For binary molecular compounds, usethe following to determine the orderof the nonmetal or metalloid elementsin the formula:
Element: B Si,C Sb,As,P,N H Te,Se,S I,Br,Cl O FGroup IIIA IVA VA VIA VIIA
H-P-H is written PH3H
Practice
Selenium disulfur dichloride carbon monoxidetrioxide, SeO3 S2Cl2 CO
Practice
Nitrogen trifluoride diphosphorous oxygen difluorideNF3 tetrahydride OF2
P2H4
Figure 2.23: A flowchart for naming binary compounds. Figure 2.24: Overall strategy for
naming chemical compounds.
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– Acids are traditionally defined as compounds with a potential H+ as the cation.
Chemical Substances; Formulas and Names
• Acids
– Binary acids consist of a hydrogen ion and any single anion. For example, HCl is hydrochloric acid.
– An oxoacid is an acid containing hydrogen, oxygen, and another element. An example is HNO3, nitric acid. (see Figure 2.23)
Figure 2.25: A flowchart for naming acids. An acid is best considered as one or more H+ ions
attached to an anion.
Figure 2.23: Molecular model of
nitric acid.
HNO3
